Thanks for replies.

I am running a general (injector?) cleaner in my latest full fuel tank, I will look into Turbo Cleaner for fuel for next tankful.

I have no insider knowledge of how the FL2 records and reacts to error codes.
My past experience with computer/machinery fault testing leads me to believe that the codes are just stored in registers and counted against time, before escalating to warnings (or clearances, if below a threshold)

Therefore clearing the pending errors as I did, by OBD Reader, Double Locking the car may have reset Error Logging Systems.

I have found in the past that stopping for a bit of shopping has temporarily cleared problems in the past without any OBD Reader action.

PS These faulty periods have occurred 3 years apart with nothing happening between these times. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 84k+ miles) (MY2015)
Metro in its 11th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)

I had this problem a couple of years ago, Italian Tune-up and a run up the motorway had no effect.
It came to a head when in the Lakes, so as a last resort went to Halfords in to buy some Wynns Turbo cleaner. I have to admit to being very skeptical but after adding it to the tank, the car went into limp mode once on the return from Halfords and hasn't done it since (fingers crossed).
What I did notice is that the exhaust made that "tinkling" noise as it cools after switching off the engine, indicating that it is running hotter and I understand this is what removes the carbon build-up.
Since then I have been using only Shell premium rather than bio diesel and notice this very same effect.